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Soviet Journal of Quantum Electronics

Resonator-free amplification of spontaneous radiation under recombination pumping conditions

A. L. Belov, F. V. Bunkin, Sergei I Yakovlenko

Published Jan 31, 1987• 0 citations
Publisher

Abstract

A one-dimensional radiative transfer model is used in a study of the amplification of spontaneous radiation as a result of a population-inverted ionic transition pumped by recombination. The problem of such transfer is solved analytically in the steady-state case and it is shown that the intensities of the radiation emitted in both directions are the same irrespective of the distribution of the unsaturated gain over the length of the active medium. Numerical calculations are reported of the stage of establishment of light fluxes. It is shown that during this transient stage a nonlinear interaction between opposite light fluxes is responsible for nonmonotonic behavior of the output radiation intensity. The problem of amplification in a traveling pump wave is treated analytically.

DOI
10.1070/qe1987v017n01abeh006365

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A one-dimensional radiative transfer model is used in a study of the amplification of spontaneous radiation as a result of a population-inverted ionic transition pumped by recombination. The problem of such transfer is solved analytically in the steady-state case and it is shown that the intensities of the radiation emitted in both directions are the same irrespective of the distribution of the unsaturated gain over the length of the active medium. Numerical calculations are reported of the stage of establishment of light fluxes. It is shown that during this transient stage a nonlinear interaction between opposite light fluxes is responsible for nonmonotonic behavior of the output radiation intensity. The problem of amplification in a traveling pump wave is treated analytically.
A new version of obtaining short-wave lasing on transition between highly excited states and the ground state of active atoms in the buffer gas atmosphere is studied theoretically. The mechanism of obtaining population inversion on such a transition is associated with the establishment of the local Boltzmann distribution of populations in a group of highly excited levels due to frequent collisions. If the excitation of the upper-lying level is performed by two laser radiation sources with frequencies ω 1 and ω 2 , short-wave lasing can be obtained at a frequency close to the total frequency ω 1 + ω 2 . The conditions for the emergence of population inversion are analyzed and simple analytic formulas are derived. It is shown that collision-induced enhancement of short-wave radiation can occur for pumping intensities on the order of 100 W/cm 2 . For pumping intensities on the order of 1000 W/cm 2 , the amplification factor for short-wave radiation may attain values of 3 cm −1 (for an active atom concentration of N ∼ 10 15 cm −3 ), which is sufficient for the development of lasing per path through an active medium (superradiance condition) for a length of the active medium on the order of 10 cm.
Equations describing a transient degenerate four-wave interaction in a resonant medium are derived and solved numerically. The approximation of a "following" nonlinear polarization state is used to determine the range of values of the parameters of the medium and the radiation field in which the efficiency of a transient four-wave interaction is higher than that of the steady-state process. It is found that amplification of weak radiation as a result of transient energy exchange can only be achieved in the four-wave interaction for different pump wave intensities.
Amplified spontaneous emission of laser media having a spherical or disk shape is numerically analyzed. Some properties are discussed and a comparison with linear systems is made. Analytical approximations, for the case of high pumping rates and high efficiency, are obtained. Energy level schemes of dye systems are used in the analysis and in the numerical calculations. A coupled set of radiative transfer and rate equations is solved under quasi-steady-state conditions. The model takes into account the fluorescence spectrum and gain saturation effects with radially symmetric distributions of the active dye molecular densities. As a comparison, the numerical examples also include an H2O system which is relevant in astrophysics.
We report a theoretical study on the possibility of amplification and generation of radiation by an inversionless two-level system in the ‘red’ wing of its spectral line under resonant diode pumping. The two-level system is used to simulate atoms of an active gas, while it is in the atmosphere of a high-pressure buffer gas. The effect results from the fact that the probability of stimulated emission in the ‘red’ wing of the spectral line exceeds the probability of absorption if the homogeneous broadening due to the interaction of particles with the buffer gas significantly exceeds the natural one (at high pressures of the buffer gas). It is found that the higher the buffer gas pressure and the higher the pump radiation intensity, the greater the inversionless amplification. It is shown that the gain in a two-level system in the ‘red’ wing of its spectral line can reach 0.011 cm −1 when the radiation intensity and the width of the radiation spectrum of pump diodes are 5 kW cm −2 and 4 cm −1 , respectively. The use of transverse diode pumping of an active medium placed in an optical cavity will make it possible to obtain lasing with frequency tuning and thereby solve the problem of converting incoherent broadband radiation into a coherent laser light in a gas of two-level active particles.